1. Field of the Invention
This invention relates generally to optical fiber communications, and more particularly, to the use of independent gain control for different frequency channels in an optical fiber communications systems utilizing frequency division multiplexing.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only farther intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3, OC-3, or equivalent connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable. There is a significant installed base of optical fibers and protocols such as the SONET protocol have been developed for the transmission of data over optical fibers. The transmitter converts the data to be communicated into an optical form and transmits the resulting optical signal across the optical fiber to the receiver. The receiver recovers the original data from the received optical signal. Recent advances in transmitter and receiver technology have also resulted in improvements, such as increased bandwidth utilization, lower cost systems, and more reliable service.
However, current optical fiber systems also suffer from drawbacks which limit their performance and/or utility. Many of these drawbacks are frequency dependent. For example, optical fibers typically exhibit dispersion, meaning that signals at different frequencies travel at different speeds along the fiber. More importantly, if a signal is made up of components at different frequencies, the components travel at different speeds along the fiber and will arrive at the receiver at different times and/or with different phase shifts. As a result, the components may not recombine correctly at the receiver, thus distorting or degrading the original signal. In fact, at certain frequencies, the dispersive effect may result in destructive interference at the receiver, thus effectively preventing the transmission of signals at these frequencies. Dispersion effects may be compensated by installing special devices along the fiber specifically for this purpose. However, the additional equipment results in additional cost and different compensators will be required for different types and lengths of fiber.
As another example, the electronics in an optical fiber system typically will have a transfer function which is not flat. That is, the electronics will exhibit different gain at different frequencies. In other applications, an electronic equalizer may be used to compensate for these frequency-dependent gain variations in the electronics. However, in an optical fiber system, the electronics produce an electrical signal which eventually is converted to/from an optical form. In order to take advantage of the wide bandwidth of optical fibers, the electrical signal produced by the electronics preferably will have a bandwidth matched to the wide bandwidth of the optical fiber. Hence, any electronic equalizer will also have to operate over a wide bandwidth, which makes equalization difficult and largely impractical.
Furthermore, as optical fiber systems become larger and more complex, there is an increasing need for efficient approaches to manage and control these systems. In a common architecture for optical fiber systems, the system includes a set of interconnected nodes, with data being transmitted from node to node. In these systems, there is commonly also a need for control, administrative or overhead information to be transmitted throughout the system or between nodes. Information describing the overall network configuration, software updates, diagnostic information (including both point to point diagnostics as well as system-wide diagnostics), timing data (such as might be required to implement a global clock if so desired) and performance metrics are just a few examples of these types of information.
Thus, there is a need for optical communications systems which reduce or eliminate the deleterious effects caused by frequency-dependent effects, such as fiber dispersion and the non-flat transfer function of electronics in the system. There is further a need for systems which support the efficient transmission of control and overhead information.